We follow the procedure described in [1]}. We first generate synthetic spectra using a grid of hydrogen-deficient spherically symmetric MARCS (Model Atmospheres in Radiative and Convective Scheme) atmospheric models with input compositions characteristic of RCB and dLHdC stars (log \(\epsilon \) (H) = 7.5, C/He = 0.01, [2]}, [3]}, [4]}, [5]}). We generated the synthetic spectra using the package TURBOSPECTRUM [6]}. We set log \(g\)  = 1.0 and varied the effective temperatures from 4000 to 7500 K in intervals of 250 K. We chose \(^{16}\) O/\(^{18}\) O values of 0.01, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 500 and infinity (no \(^{18}\) O). We used nitrogen abundances of log \(\epsilon \) (N) = 7.0, 7.5, 8.0, 8.5 and 9.4. For RCB stars, the warm dust shell can contribute significantly to the K-band flux (up to 80%, [7]}), filling up the absorption bands. To account for this, we introduced an additional parameter f\(_{\mathrm {dust}} = \frac{F_{\mathrm {shell,K}}}{F_{\mathrm {total,K}}}\) , and varied it between 0 to 0.8 in steps of 0.1 for RCB stars. We do not expect this to be a significant effect for dLHdC stars as they do not have infrared dust excesses. We fit the synthetic spectra to continuum-normalised NIR spectra and visually examine each of the fits to determine the range of isotope ratios that is consistent with the observed spectra. The derived values of \(^{16}\) O/\(^{18}\) O are listed in Table REF .
